STUDY OF REDOX REACTIONS FOR SOLAR THERMOCHEMICAL CYCLES by IBRAHEAM ABDULRAHMAN AL-SHANKITI B.S., University of Tulsa, 2007 M.S., University of Colorado, 2014 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Chemical and Biological Engineering 2018 This thesis entitled: Study of Redox Reactions for Solar Thermochemical Cycles written by Ibraheam Abdulrahman Al-Shankiti has been approved for the Department of Chemical and Biological Engineering _____________________________________________ Alan W. Weimer, Committee Chair _____________________________________________ Hans H. Funke, Committee Member Date _______________ The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. Al-Shankiti, Ibraheam Abdulrahman (PhD, Chemical and Biological Engineering) Study of redox reactions for solar thermochemical energy storage and H2O splitting Thesis directed by Professor Alan W. Weimer Solar thermal splitting of water or carbon dioxide is a promising technology for producing hydrogen and/or carbon monoxide. In a two-step cycle, a metal oxide is thermally reduced with concentrated solar radiation to release oxygen. The reduced metal oxide is then re- oxidized with steam or carbon dioxide to produce hydrogen or carbon monoxide. The two-step redox cycle can be operated either as a temperature swing where there is a temperature difference between the reduction and oxidation steps or isothermally. This work discusses various aspects of operating the redox cycle isothermally including redox cycle thermodynamics and overall system efficiency and describes solar reactor concepts based on isothermal operation. This work reports the reduction kinetic study of the hercynite cycle (FeAl2O4) for high temperature solar thermochemical water splitting. The reaction kinetics has been evaluated using dynamic thermogravimetric and XRD analyses. Kinetic modeling results indicate that as- prepared hercynite materials undergoes reduction via two different reaction mechanisms. The reaction first proceeds by a nucleation and growth reaction mechanism, followed by a third-order kinetic model. XRD analyses show the occurrence of superstoichiometric oxygen in the spinel structure of FeAl2O4+δ in the second reaction mechanism, which indicates the formation of cationic vacancies. TGA and XRD analyses show that hercynite materials operates via a cation- vacancy mechanism when the materials are thermally reduced and oxidized with steam. High-temperature thermochemical energy storage shows promise in aiding concentrating solar power plants in meeting variable, grid-scale electricity demand. In this work, manganese iii oxide-based mixed metal oxide particles have been designed and tested for high temperature solar thermochemical energy storage. We evaluate the effects of Al2O3, Fe2O3, and ZrO2 in Mn2O3-based spray-dried particles in a TGA between 650°C and 1,200°C over six consecutive redox cycles. Results are compared with thermodynamic predictions from 400–1,400°C under oxidizing and reducing atmospheres. A mixture of 2:1 Fe2O3:Mn2O3 formed iron manganese oxide spinel (MnFe2O4) on calcination, and demonstrated the highest thermochemical activity. Conversely, zirconia was an inert support that does not react with manganese oxide. The oxidation reaction kinetics of MnFe2O4 has been evaluated using solid-state kinetics theory and XRD analysis. A kinetics study indicates that the reaction proceeds by two different reaction mechanisms. The reaction first proceeds by a diffusion-controlled reaction mechanism with no phase change, followed by a nucleation-growth reaction mechanism. iv This thesis is dedicated to my beloved family v ACKNOWLEDGMENTS This work would not have been possible without the help and support of my advisor, Professor Alan Weimer. He has been a steadfast guide for my journey through my graduate studies. I am also grateful to Team Weimer for their help and support. Many members have contributed to this work. Brian Ehrhart has worked very closely with me and has always been helpful. I take pride in being part of this amazing research group. Research would not be possible without financial support. I would like to acknowledge the financial support toward the research from U.S. DOE Office of Energy Efficiency and Renewable Energy (EERE), Fuel Cell Technologies Office under Award Number DE-EE0006671. The Saudi Basic Industries Corporation (SABIC) also has financially supported me to complete my graduate studies. Hicham Idriss, corporate researcher, offered invaluable support from the beginning of my graduate studies. Thank you Hicham. Nobody has been more important to me in the pursuit of this project than the members of my family. I would like to thank my parents Abdulrahman and Maryam. They have taught me to learn and be curios. Thank you very much. My siblings have been very supportive. Finally, I want to thank my wonderful wife Khawla who have been so helpful, patient, and kind even when I have been consumed with this work. There are not enough words to describe her support for me. Thank you and I love you. vi CONTENTS 1. INTRODUCTION ................................................................................................................... 1 1.1. Motivation ........................................................................................................................ 1 1.2. Solar Thermochemical Water Splitting ............................................................................ 1 1.3. Two-Step STWS Active Materials ................................................................................... 3 1.3.1. Thermodynamics of STWS Materials ...................................................................... 4 1.3.2. Current STWS Materials........................................................................................... 5 1.4. Project Objectives .......................................................................................................... 15 1.5. References ...................................................................................................................... 16 2. ISOTHERMAL REDOX FOR H2O AND CO2 SPLITTING – A REVIEW AND PERSPECTIVE............................................................................................................................. 24 2.1. Abstract .......................................................................................................................... 24 2.2. Introduction .................................................................................................................... 24 2.3. Chemical thermodynamics ............................................................................................. 25 2.4. Isothermal vs. non-isothermal ........................................................................................ 29 2.5. Solar to H2/CO thermodynamic process efficiency model ............................................ 32 2.5.1. Materials and kinetics effect ................................................................................... 34 2.5.2. Reduction reaction processing ................................................................................ 35 2.5.3. Heat exchanger effectiveness .................................................................................. 38 2.6. Isothermal reactor designs .............................................................................................. 42 2.6.1. Monolithic-receiver reactors ................................................................................... 42 2.7. Summary and path forward ............................................................................................ 45 2.8. References ...................................................................................................................... 46 vii 3. UNDERSTANDING REDUCTION KINETICS OF HERCYNTE MATERIALS FOR SOLAR THERMOCHEMICAL H2O SPLITTING ..................................................................... 49 3.1. Abstract .......................................................................................................................... 49 3.2. Introduction .................................................................................................................... 49 3.3. Experimental methods .................................................................................................... 51 3.3.1. Materials preparation .............................................................................................. 51 3.3.2. Kinetic studies ......................................................................................................... 51 3.3.3. Materials characterization ....................................................................................... 53 3.4. Results and discussions .................................................................................................. 54 3.4.1. Experimental TGA results ...................................................................................... 54 3.4.2. Kinetic modeling ..................................................................................................... 55 3.4.3. Reduction reaction mechanism with H2O splitting ................................................. 63 3.5. Conclusions ...................................................................................................................